Variation of RKKY interaction strength in dilute AuFe alloys

From measurements of the magnetic properties of some dilute AuFe alloys we find that V₀, the strength of the Ruderman-Kittel-Kasuya-Yosida interaction, V(r) = (V₀ cos 2k_Fr)/r³, decreases rapidly from V₀ = 11.9 × 10^-36 erg cm³ at n = 42 ppm Fe to 1.03 × 10^-36 erg cm³ at 6050 ppm Fe. We suggest that the observed decrease of V₀ is due to self-damping of the RKKY oscillations, and discuss the significance of this decrease for the interpretation of other experiments on AuFe.     

The Kondo temperature of gold

Precise measurements of the low temperature electrical resistivity were obtained for 7 samples of highly dilute AuFe alloys. The effective Fe concentrations ranged between 0.006 and 0.2 ppm. At these low concentrations no impurity-impurity effects could be detected and the data were in good agreement with the Hamann theory. Values of the impurity spin S and the Kondo temperature T_K were determined from the Hamann equation to be (0.15±0.01) and (1.5±0.1) K respectively.     

Effect of mean free path on the strength of the Ruderman-Kittel-Kasuya-Yoshida interaction in a dilute (Ag0.6Au0.4)Mn alloy

From measurements of magnetic susceptibility from 1.2 to 200 K and magnetization (0–50 kG) from 1.2 to 4.2 K, the strength of the RKKY interaction between Mn spins in a Ag_0.6Au_0.4 + 810 ppm Mn alloy has been determined to be V₀ = (2.4±0.5) x 10^-37 erg cm³. It is shown that this value for V₀ is consistent with an RKKY interaction strength which has has been reduced by mean free path effects by a factor exp (— 〈 r〉/l), where 〈 r 〉 is the average separation between Mn spins and l is the electron mean free path in the alloy.     

Anisotropic transferred hyperfine fields acting on 119Sn in gadolinium metal and alloys

Mossbauer studies of 0.1% ¹¹⁹Sn in gadolinium alloys at 4.1 K were performed. The spectra were well reproduced by a superposition of hyperfine fields according to the RKKY model with k_F = 1.27 ± 0.04 A^-1. In Gd_0.95Tm_0.05 where the magnetization is along the c-axis, the transferred field per Gd ion is 4 ± 1% lower than in Gd_0.97Tb_0.03 where the magnetization is perpendicular to the caxis.     

Resistance maximum across the spin glass to Kondo transition

In a recent theory of the noise model of alloys like AuFe a singular point at zero temperature was found to separate a spin glass phase at high concentrations and a Kondo phase at low concentrations. Despite this there is a resistance maximum in both “phases”, although of different characters. In the present letter a relation is given between the temperature of the maximum, T_m, the noise temperature, Δ_c, and the Kondo temperature, T_K. This extends a previously given expression, that is only valid in the spin glass limit Δ_c >> T_K, across the transition at Δ_c = T_K into the Kondo phase and values of Δ_c less than T_K.     

The specific heat of CeAl2 between 0.02 and 1 K

We report specific heat measurements on a CeAl₂ single crystal between 0.02 and 1 K. Above 0.08 K, we found C₀ = γT + βT³ with γ = (130±0.5) mJ/K²mole and β = (142±1) mJ/K⁴mole in good agreement with previous results above 0.3 K. Below 0.08 K, an excess specific heat C_N = αT^?2 with α = (6.4±1) mJK/mole was detected and interpreted in terms of hyperfine splitting of the Al²⁷ nuclear states. Our results suggest that in CeAl₂ (complex) antiferromagnetism coexists with the Kondo effect at least down to 20 mK.     

The Ruderman-Kittel-Kasuya-Yosida interaction in amorphous La80Au20 alloys with dilute Gd impurities

From magnetization measurements on some amorphous dilute La_80?xGd_xAu₂₀ alloys with x ? 1 we have shown that the magnetic behavior follows the scaling laws of a spin-glass system, characteristic of the 1/r³ dependence of the pairwise interaction. We have also determined the strength of the Ruderman-Kittel-Kasuya-Yosida interaction V(r) = (V₀cos 2k_Fr)/r³, to be V₀ = 0.20 × 10^?37 ergcm³. The corresponding value of the s-f exchange integral is |J_sf| = 0.14 eV, which is compared with values determined from other experiments.     

Effect of substitution of Ce by La in the CeNixPt1 − x dense Kondo ferromagnets

The effects of substitution of Ce by La in the orthorhombic CeNi_{1 − x}Pt_x dense Kondo ferromagnets are studied by means of magnetization and electrical resistivity measurements. A decrease of the exchange RKKY interactions leads to a decrease of the Curie temperature T_c as a function of the La content and hence to an enhancement of the Kondo character in the thermal dependence of the resisitivity. However, the Ce moment is almost independent of the La amount. The Kondo temperature being also independent, this surprising result seems in contradiction with the available Kondo lattice models.     

Mössbauer study of magnetic ordering in K2FeF4

The sublattice magnetization of the quadratic-layer basal-plane antiferromagnet K₂FeF₄ has been studied by use of Mössbauer spectroscopy. A distribution of Néel temperatures with a width ～3 K is found at a mean T_N = 67.2±0.3 K. For 0.3 < T/T_N < 0.99 the sublattice magnetization is described by a power law with critical exponent β = 0.17±0.01.     

Long range interactions and spin wave excitations in FexPd82−xSi18 metallic glasses

The low temperature magnetization of amorphous Fe_xPd_82?xSi₁₈, 2 ? x ? 25, approaches saturation like 1/H. RKKY-type cluster-cluster interactions and single impurity effects are indicated. The effective strength of the RKKY interaction decreases with increasing x. The magnetization of both spin glass-like and weakly ferromagnetic alloys exhibits Bloch's T^{$\text{3}\text{2}$} dependence. The spin wave stiffness constants increase linearly with 2 ? x ? 25. The range of exchange interactions decreases with increasing x.     

Magnetic characteristics of MgFe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanoparticles obtained by glycine–nitrate synthesis

The magnetic properties of magnesium–iron spinel (MgFe₂O₄) powdered nanoparticles obtained by glycine–nitrate synthesis are investigated by X-ray phase analysis and the NMR method. According to the results of X-ray phase analysis, the average size of the crystalline part of nanoparticles of the powder under investigation is 45 ± 4 nm. Magnetization J is determined using the formula J = (B/μ0)–H, where B and H are the induction and strength of the magnetic field in the sample, which are measured by the NMR method. The magnetic characteristics of MgFe₂O₄ are as follows: specific saturation magnetization J_sat = 17.52 A m²/kg, specific residual magnetization J_r = 5.73 A m2/kg, coercive force H_c = 4600 A/m, and magnetic moment P_sat = 371 × 10^–20 A m² in the magnetic saturation state and P_r = 121 × 10^–20 A m² in the residual magnetization state.     

Determination of the deuteron mean square radius

Three new methods are applied to the problem of extracting the deuteron rms matter radius from the experimental ratio of (e, d) to (e, p) scattering. A new value r_Ed = 1.953 (3) fm is obtained. The asymptotic method also yields 〈r⁴〉 = 54.5 ± 0.3 fm⁴ and 〈r⁶〉 = 1914 ± 20 fm⁶. Potential models of the deuteron are apparently unable to explain this r_Ed simultaneously with the low energy effective-range parameters of the neutron-proton system.     

On some low-temperature properties of (La, Ce)B6 alloys

Some recent experimental data on the magnetic susceptibility and the low-temperature magnetization of the (La, Ce)B₆ system are discussed in the light of the theoretical results. A value of the s-f exchange constant J is estimated which is also consistent with the results obtained from both the Kondo effect and the RKKY interaction.     

Magnetization reversal in Fe48Pt34B34 magnetic foils

Exchange-spring magnet L1-FePt/(Fe₂B+α-Fe) is fabricated by flash annealing a melt-spun Fe₄₈Pt₃₄B₃₄ foil. A coercivity of 8500 Oe (1 Oe = 79.5775 A/m), squareness (M_r/M_s) of 0.70, saturation magnetization of 10.2 kGs (1 Gs = 10^-4 T) and an effective anisotropy K_eff =2.0 × 10⁷ ergs/cm³ are obtained. A two-step magnetization reversal feature is characterized in this paper. An exchange bias phenomenon is also observed in a low saturation field.     

Revision of the109Cd decay data

Combining measurement with the use of a Si(Li) detector and a windowless 4π NaI(T1) detector a new value ofω _K(Ag) equal to 0.84±0.02 has been determined. From the measured ratio of the KX-ray and conversion electron emission rates the relative K-capture probabilityP _K=0.77±0.015 was calculated. Through analysis of data reported in literature and taking into account the new value ofP _K andω _K and the results of the international comparison of absolute activity measurements of¹⁰⁹Cd, the following decay data were chosen for the calculation of yields:ω _K=0.836±0.007;α _T=26.4±0.3; α_K=11.4±0.3; P_K=0.78±0.015. The number of gamma photons per disintegration (gamma-yield) was then determined equal to 0.0365±0.0004; the KX-yield equal to 0.997±0.008; the conversion electrons yield equal to 0.9635±0.0005 and the KX/γ ratio equal to 27.3±0.6.     

Internal rotation and molecular structure of ethaneselenol by microwave spectroscopy

The microwave spectra of ethaneselenol and its deuterated and ¹³C-substituted species were measured and assigned for the gauche and trans isomers. The double minimum splittings in the gauche isomers were directly observed for the species having a symmetry plane in the frame part. The rotational constants and the torsional splitting of the gauche isomer of the parent species were determined to be A = 27 148.86 ± 0.05, B = 3 623.68 ± 0.01, C = 3 399.21 ± 0.03, and Δν = 1 083.33 ± 0.04 MHz. From the torsional splittings of the parent and SeD species together with the vibrational frequencies already reported by Durig and Bucy, the Fourier coefficients of the selenol internal rotation potential function were determined to be V₁ = ?44 ± 17, V₂ = ?260 ± 3, V₃ = 1202 ± 16, and V₆ = ?43 ± 9 cal/mole. From the rotational constants obtained, the r_s structural parameters of the gauche and trans isomers were determined. The structural parameters in the skeletal part for the gauche isomer are $\text{r}\text{(CC) = 1.524}\text{A}\text{?}$, $\text{r}\text{(CSe) = 1.957}\text{A}\text{?}$, $\text{r}\text{(SeH) = 1.467}\text{A}\text{?}$, α(CCSe) = 113°31′, α(CSeH) = 93°05′, and the dihedral angle τ(CCSeH) = 61°39′. Those for the trans isomer are $\text{r}\text{(CC) = 1.525}\text{A}\text{?}$, $\text{r}\text{(CSe) = 1.962}\text{A}\text{?}$, $\text{r}\text{(SeH) = 1.440}\text{A}\text{?}$, α(CCSe) = 108°43′, and α(CSeH) = 93°30′. These parameters were compared with the corresponding ones of ethanethiol.     

Hall effect in CeAl3

We have studied temperature dependences of the Hall coefficient R_H(T) and resistivity ?(T) in substitutional solid solutions Ce_xLa_1?xAl₃ (0 ≤ x ≤ 1) which realize the transition from Kondo impurity regime (X?1) to Kondo lattice (X = 1). Lowering temperature results in the anomalous increase of R_H(T). The ratio r = R_H(3 K)/R_H(100 K) has been found to increase with cerium content up to the r ? 15 in the marginal compound CeAl₃. The specific features of R_H(T) behaviour in Ce_xLa_1?xAl₃ alloys seem to be related to the existence at low temperatures of heavy fermions due to the formation of the narrow Abrikosov-Suhl resonance at the Fermi level.     

Molecular-field theory of moment reduction in Kondo systems with crystal-field effects

Several dense Kondo compounds have a low-temperature ordered phase in which the magnetic moments are reduced with respect to the values expected for the crystal-field (CEF) ground state. In the present work the phenomenon of moment reduction is studied within a molecular-field theory combined with a variational solution of the one-impurity Anderson model with CEF effects. The calculated zero-temperature magnetization and susceptibility agree well with available exact results; the present method is easily applied to systems of any symmetry. We first study the f ¹ configuration in cubic symmetry, for small values of the ratio T _K/Δ between Kondo temperature and CEF splitting. With a Γ _∞ ground state and a field along a 〈100〉 direction, an inflection point occurs in the magnetization curve, which gives rise to a first order transition in the zero-temperature phase diagram. This feature is not found for a field along 〈110〉 or 〈111〉, for which the transition is second order. For a Γ ₇ ground state and small values of T _K/Δ, the magnetic-nonmagnetic transition is second order for all field directions. On increasing T _K/Δ an inflection point in the magnetization curve appears first for a field along 〈111〉. The theory is applied to a study of cubic CeAg, CeAl₂, CePb₃, CeIn₃, CeTe, and hexagonal CePd₂Ga₃. The bare value of the moment is found to be strongly increased by exchange coupling to excited CEF states, and the amount of Kondo reduction is found to be substantial, confirming the importance of the Kondo effect in these compounds.     

Doping effects on the magnetic properties of NdRhIn5 intermetallic antiferromagnet

We report temperature dependent heat capacity and magnetization measurements on single crystals of Nd_1-xLa_xRhIn₅ (x=0.15, 0.4 and 0.5) and NdRhIn_5-xSn_x (x=0.08, 0.12 and 0.24). NdRhIn₅ is an antiferromagnetic (AFM) compound with T_N≈ 11 K which crystallizes in the same layered tetragonal structure of the CeMIn₅ family (M=Rh, Co and Ir), where different ground states can be found by tuning the interplay among different microscopic interactions such as the Kondo effect, crystal field (CEF) effects and the Ruderman-Kittel-Kasuya-Yoshida (RKKY) magnetic interaction. Here, we explore the evolution of the AFM correlations in this Nd-based (non-Kondo) compound while perturbing the RKKY exchange by using two different substitutions: (i) replacing Nd³⁺ by non-magnetic La³⁺ within NdIn₃ atomic planes (dilution) and (ii) substituting In by Sn in the In-sites (electronic tuning). For both types of doping, our results show the suppression of the AFM state as the La- or Sn-content is increased. This doping induced suppression of the AFM order is discussed considering the effects of dilution and effects in the tetragonal CEF using a mean-field model applied to the observed data. Our results are compared to the properties of other members of the RRhIn₅ family considering the role of dimensionality in the magnetic interactions.     

Kondo effect and impurity-impurity interaction in (La,Ce)B6 alloys

Measurements of the magnetic susceptibility between 0.03 and 300 K and of the magnetization between 0.05 and 10K for magnetic fields up to 60kOe have been used to investigate effects from the interaction between the conduction electrons and local magnetic moments in (La_1–x Ce _x )B₆ alloys (0.0007x0.10). For Ce concentrationsx<0.006 the data show Kondo-type single impurity behavior at low temperatures with a transition from a magnetic to a non-magnetic regime of the Ce ions. In the magnetic regime the impurity susceptibility follows a Curie-Weiss law, and in the non-magnetic regime it varies withT ². An external magnetic field gradually restores the free-ion behavior of the Ce impurities. For more concentrated alloys interactions between the impurities are observed. The RKKY interaction strength derived is more than two orders of magnitude smaller than in the Kondo systemCuFe. Values of thes–f exchange integral,J, estimated from both the Kondo effect and the RKKY interactions are in good agreement. The relatively high Kondo temperature, in spite of a smallJ, can be explained by a density-of-states argument. The influence of crystal field excitation on the susceptibility is also discussed.